1. Field of the Invention
The present invention relates to a substrate with marker provided with a marker for adjusting an irradiation position of a laser beam, and a manufacturing method of the substrate. Moreover, the present invention relates to an irradiation method of a laser beam using the substrate with marker, a laser irradiation apparatus, and a light exposure apparatus. Further, the present invention relates to a manufacturing method of a semiconductor device with the use of the laser irradiation apparatus or the light exposure apparatus.
2. Description of the Related Art
In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has drastically progressed and development for applying a TFT to an active matrix display device has been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher electric field effect mobility (also referred to as mobility, simply) than a conventional TFT using a non-single crystal semiconductor film, high speed operation is possible. Therefore, it has been attempted that a pixel, which has been conventionally controlled by a driver circuit provided outside a substrate, is controlled by a driver circuit provided over the same substrate as the pixel.
A substrate used for a semiconductor device is expected to be a glass substrate rather than a single crystalline semiconductor substrate in terms of cost. However, a glass substrate is inferior in heat resistance and easy to be deformed due to heat. Therefore, when a TFT using a polycrystalline semiconductor film is formed over a glass substrate, laser annealing is often employed for crystallizing a semiconductor film in order to prevent the glass substrate from being deformed due to heat.
Compared with another annealing method which uses radiation heat or conduction heat, the laser annealing has advantages such that the process time can be shortened drastically and that a semiconductor substrate or a semiconductor film can be heated selectively or locally so that thermal damage is hardly given to the substrate.
In this specification, the laser annealing means a technique of crystallizing a damaged layer or an amorphous layer formed in a semiconductor substrate or a semiconductor film, a technique of crystallizing an amorphous semiconductor film formed over a substrate. Moreover, the laser annealing includes a technique applied to planarize or modify a surface of a semiconductor substrate or a semiconductor film.
Laser oscillators used for the laser annealing can be broadly classified into pulsed laser oscillators and continuous wave (CW) laser oscillators according to the oscillation method. In recent years, it has been known that the size of a crystal grain formed in a semiconductor film becomes larger when using a CW laser oscillator such as an Ar laser or a YVO4 laser than when using a pulsed laser oscillator such as an excimer laser at the crystallization of the semiconductor film. When the size of the crystal grain in the semiconductor film becomes larger, the number of grain boundaries in a channel forming region of a TFT formed using this semiconductor film decreases; therefore, the mobility increases. Accordingly, thus manufactured TFT can be used to develop a more sophisticated device. This is the reason why the CW laser oscillator attracts attention.
In general, a beam spot of a laser beam (also referred to as laser light) used for laser annealing of a semiconductor film has a linear shape, and laser annealing is performed by scanning the beam spot of the laser beam processed into a linear shape over a semiconductor film. When the beam spot of the laser beam is processed into a linear shape, the semiconductor film can be crystallized efficiently.
In the case where a silicon (Si) film with thicknesses of several tens to several hundreds of nm which is generally used for a semiconductor device is crystallized with the use of a YAG laser or a YVO4 laser, a laser beam of a second harmonic with shorter wavelength than the fundamental wave is used in terms of an absorption coefficient. This is because the semiconductor film can be crystallized more efficiently, as an absorption coefficient of laser light with respect to the semiconductor film is larger.
In general, in a step of performing laser annealing using a continuous wave laser oscillator with a harmonic, a semiconductor film is annealed nonuniformly. This is because laser light emitted from a continuous wave laser oscillator generally has Gaussian energy distribution.
In opposite ends of a linear beam in a longitudinal direction, a region with extremely small crystal grains and poor crystallinity compared with that in the center thereof is formed. This is because, since the opposite ends of the linear beam have low energy, there is a tendency that a region which is not melted partially remains, such crystal grains with a large grain diameter that are formed in the vicinity of the center of the linear beam cannot be obtained, and crystal grains with a comparatively small grain diameter (microcrystals) are formed in the region. Unevenness is noticeable in a surface of a region in which microcrystals are formed, which is unsuitable for manufacturing a semiconductor element. In the case where a semiconductor element is formed over the region, it causes variation in electric characteristic or operation failure. That is, in a step of performing laser annealing, it is necessary to decide in advance in which area over the substrate a semiconductor element is formed and to perform laser annealing by targeting a region in which a semiconductor element is formed.
In general, the size of a semiconductor element is the square of several μm to several hundreds of μm, which is extremely minute. Further, a length of the beam spot of the linear laser beam is as short as several hundreds of μms to several mm. Therefore, it is necessary to decide a position of laser irradiation extremely precisely when crystallization is performed by targeting a region in which a semiconductor element is formed. Accordingly, in performing laser irradiation, a marker is formed in a desired position over a substrate and precise alignment is performed using the marker as a reference point; thus, laser irradiation is performed (for example, Patent Document 1: Japanese Published Patent Application No. 2003-224084). It is to be noted that a method for recognition by processing an image captured by a CCD camera is frequently used for precise alignment.
In manufacturing a semiconductor element, a method of precise alignment using a marker as described above is generally used in, for example, a light exposure step of a photolithography method or a laser direct writing step used for formation of a laser semiconductor element, division, opening, and the like, besides a laser annealing step.